Cryogenic tanks are utilized to store a fluid at cryogenic temperatures of typically less than 100° K and even at less than 30° K at pressures in the range of about 1 to 12 bars. Such tanks allow for the efficient storage of fluids that are normally gaseous at ambient temperature and/or pressure. The fluid will be in a two-phase state having a liquid portion and a gaseous portion that vary with the quantity of fluid within the tank, the pressure in the tank and the temperature in the tank.
Currently, the quantity of the fluid having a dielectric property within cryogenic storage tanks is ascertained by the use of a capacitive probe. The capacitive probe has an inner tube that is positioned inside an outer tube with a small annular gap therebetween. The probe is open at both ends with the fluid filling the space within the annular gap. Each of the tubes are electrical conductors and the dielectric constant of fluid therebetween will change with the makeup (liquid phase and gas phase) of the fluid filling the annular gap. The value of the capacitance varies with the dielectric constant and is indicative of an average density of the fluid in the tank, thus yielding the quantity of the fluid within the tank.
To use these probes, lead wires are applied to the inner and outer tubes and are routed through existing piping to a control module that measures the capacitance and is located outside of the storage tank. Typically, these wires will be at least one meter or more in length. Additionally, these lead wires are coaxial shielded wires, which also have an intrinsic capacitance.
When the fluid is hydrogen, the change in the value of the dielectric constant between hydrogen gas and hydrogen liquid is very small. For example, the dielectric constant for gaseous hydrogen is approximately 1.1 while the dielectric constant for liquid hydrogen is approximately 1.2. Thus, there is less than a 10% change in value between the dielectric constant of gaseous and liquid hydrogen. This small differential makes it difficult for the capacitance probe to accurately measure variations in the dielectric constant.
The capacitance of the connecting wires in addition to the capacitance of the probe can further reduce the percentage change in the value of the capacitance between maximum and minimum values (entirely gaseous or entirely liquid hydrogen within the tank). Thus, the probe attempts to generate accurate measurements from relatively small variations in capacitance, which limits the accuracy of the average density measurement.
Hydrogen is extracted from the tank through the gas extraction line. The extraction of the hydrogen changes the temperature in the extraction line. If the coaxial wires are routed through the extraction line, the temperature of the coaxial wires will also change. This may cause the capacitance of the coaxial wires to change and further limit the accuracy of the measure of hydrogen within the tank. The changing temperature may also cause stability problems or variations in the signal being generated therefrom, making an accurate measurement difficult to obtain. Furthermore, since the variation of the capacitance of the probe cannot be linearized to account for the tank geometry, the control module must provide compensation.